1. Field of the Invention
This invention relates to methods of removing unwanted impurities from water. In particular, this invention relates to methods of removing unwanted hydrocarbons from water, especially low molecular weight monocyclic aromatic hydrocarbons, oils and greases.
2. State of the Art
Oilfield-produced waters, remediation site waters, contaminated ground waters and refinery runoff streams all may contain high concentrations of hydrocarbons. These petroleum processing-derived waste waters contain particularly high concentrations of several classes of organic compounds due to the high water solubility of these classes of compounds. The low molecular weight monocyclic aromatic hydrocarbons, and some other structurally related non-hydrocarbon monocyclic aromatic compounds, for example, benzene, toluene, ethylbenzene, and the three isomers of xylene, i.e., orthoxylene, metaxylene and paraxylene, (a mixture of at least three of the above named components dissolved in waste water will hereinafter be referred to as BTEX), comprise one such class of relatively highly water soluble hydrocarbons. Other common dissolved contaminant hydrocarbons or organics include styrene, thiophenes, and pyridine. However, other less soluble hydrocarbons, such as aliphatic hydrocarbons found in diesel and jet fuels and in gasoline, also pollute water and must be removed. Because of health concerns, regulators have reduced the maximum acceptable concentration of BTEX in water to very low concentrations, typically &lt;5 ppmw. Consequently, producers of BTEX-contaminated water need to remove or greatly reduce the concentrations of all these polluting hydrocarbons in produced aqueous runoff streams.
Several technologies allow treatment of water to reduce the concentrations of BTEX and other hydrocarbons. These technologies include UV/ozone oxidation, UV/peroxide oxidation, high intensity UV destruction, powdered activated carbon adsorption coupled with biological treatment, granular activated carbon adsorption, air/gas stripping followed by volatile organic carbon (VOC) capture, various membrane processes, and supercritical water oxidation. While all of these technologies work acceptably well, they all require a fairly large physical plant to treat water produced on an industrial scale. For that reason, space-critical producers, for example, off shore oil platforms and the like, require different techniques to process the water they produce. A need exists for an effective water remediation method easily usable in space-critical areas.
Countercurrent technology, primarily used in high flow rate applications, presents a candidate for a small, space-critical water remediation unit since countercurrent installations typically have a small "footprint", that is, they are installations that require little plant area. Conventionally, countercurrent units soften and deionize water. In these conventional units, a countercurrent ion exchange resin removes inorganic components, for example, cations such as magnesium, iron or calcium, or inorganic anions. However, no countercurrent adsorption method has been designed to remove organic components, such as BTEX, diesel and gasoline components, using powdered adsorbent(s) suspended or bedded in essentially an aqueous fluid.
It would be advantageous to have a small footprint countercurrent unit that removes BTEX and other hydrocarbons from water to a maximum concentration of less than 5 ppmw, and particularly at relatively high flow rates, e.g., above 10 gallons per minute per square foot of adsorbent bed. The inventor has found that countercurrent technology can provide a unit that removes at least 75% of the total BTEX from a waste water stream. The unit also provides a means to remove other, non-aromatic hydrocarbons from a waste water stream.
In my co-pending patent application U.S. Ser. No. 07/996,310, highly effective hydrophobic powdered adsorbents are used in an exchange (adsorption) zone in a countercurrent water remediation method to remove BTEX. The deactivated or spent adsorbents (containing adsorbed BTEX) are continuously regenerated and then recycled to the exchange zone. Furthermore, the waste water stream may also contain water-insoluble hydrocarbons, such as crude oils, greases, lube oils, sludges, heavy diesel ends, other oils, and the like. Such water-insolubles can also be removed from the waste water stream by using the highly effective hydrophobic adsorbents in the countercurrent method. However, problems have arisen during regeneration of the spent adsorbents containing both BTEX and the water-insolubles. A need exists for an effective water remediation method when the waste water contains dissolved contaminant hydrocarbons usually including BTEX, and water-insoluble oils and greases.